Process for bleaching wood pulp with fluorine,hydrofluoric acid,and oxygen difluoride



United States Patent 3,481,827 PROCESS FOR BLEACHING WOOD PULP WITH FLUORINE, HYDROFLUORIC ACID, AND OXY- GEN DIFLUORIDE Arthur M. Brooks, Baltimore, Md., assignor to W. R. Grace & Co., New York, N.Y., a corporation of Connecticut No Drawing. Continuation-iu-part of application Ser. No.

582,177, Aug. 25, 1966. This application Aug. 2, 1968, Ser. No. 749,620

. Int. Cl. D2lc 9/12 US. Cl. 162-63 5 Claims ABSTRACT OF THE DISCLOSURE CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of US. Ser. No. 582,177, filed Aug. 25, 1966. i

BACKGROUND OF THE INVENTION Field of the invention This invention relates to processes of bleaching fibrous materials. In one aspect, the invention relates to processes of bleaching wood pulp and natural fibers, such as are used in the processes for making paper, and the like. In a further aspect the invention relates to a process for the combined bleaching and removal of siliceous compounds fromthe nodes of grasses such as bagasse and the like. In another aspect, the invention relates to a bleaching process in which pollution caused by waste liquors and wash waters can be more readily removed by subsequent treatments.

In a further aspect, my invention relates to a combined bleaching and pollution treating process which substantially reduces or eliminates pllution caused by waste liquors and wash waters.

THE PRIOR ART The prior art processes of bleaching wood pulp and other cellulosic fibers to high whiteness and brightness using chemical bleaching agents such as chlorine or hypochlorite are frequently accompanied by deleterious fiber degradation or deterioration during the bleaching. Current bleaching technology, particularly applied to the bleaching of chemical wood pulp for paper, uses chlorine, caustic soda, calcium andsodium hypochlorites, chlorine dioxide, sodium and hydrogen peroxides. With groundwood pulp, reducing agents, such as zinc or sodium hydrosulphite may be used, with or without the use of hydrogen peroxide. In the usual conventional processes, several chemicalsare used in series or stages, with residues from the reaction usually being washed out between the stages. In spite of the large amount of work that has been done in this field, and the large number of multi-stage bleaching processes that have been developed, there is still no completely satisfactory answer to the problem of bleaching wi'hout some fiber degradation.

Further, the wash waters, particularly from the bleaching of wood pulps, present a disposal and pollution problem since they usually contain unexhausted bleaching WERE-05 Patented Dec. 2, 1969 chemicals as well as the materials removed from the pulp by the bleaching process.

The prior art has discovered that chlorine dioxide used in one or more stages has the property of bleaching without substantial degradation, but chlorine dioxide is quite costly and requires special handling due to its explosive tendency. Thus, chlorine dioxide does not represent a completely satisfactory solution.

The use of hydrofluoric acid to remove adventitious and adhering sand from cotton linters is known tothe art as described in US. Patent 1,911,030. However, the use of fluorine or fluorine compounds to effect combined bleaching and removal of siliceous compounds which are internally formed within or asa part of the nodes of grasses such as bagasse is to my knowledge heretofore unknown.

Thus, it would be very desirable to provide a bleaching process which would alleviate both the fiber degradation and pollution problems of the prior art.

OBJECTIVES AND SUMMARY OF THE INVENTION Objects It is therefore an object of my invention to provide improved bleaching processes and also to provide bleaching processes which facilitate pollution control.

It is a further object of my invention to provide improved bleaching processes which cause substantially less fiber degradation.

It is a still further object of my invention to provide bleaching processes which yield fibers having increased brightness.

It is another object of my invention to provide bleaching processes using fluorine or a fluorine-generating compound as a bleaching agent for fibers.

It is also an object of my invention to provide a process for the combined bleaching and removal of siliceous compounds from the nodes of} grasses such as bagasse, wheat straw, esparto grass and the like.

It is still another objeet of my invention to provide bleaching processes wherein the wash waters and waste liquors from the bleaching process can be handled in a manner to eliminate or markedly reduce the pollution of water courses to which they are usually run, or the pollution of the general environment.

Another object of my invention is to provide combined fiber bleaching and pollution removal processes.

Additional objects will be apparent, and in part obvious, from the following description and appendant claims.

Summary In summary, I have discovered that these and other objects can be obtained by the use of fluorine and/or the reaction product of fluorine compounds, as a bleaching agent. I have also discovered that a very substantial increase in brightness can be secured in bleaching fibrous materials, such as for example, wood pulp, by treating it with fluorine compounds or gaseous fluorine and further that this bleaching is accomplished with very little fiber degradation. While my invention is particularly advantageous for the bleaching of wood pulp, it is also well suited for the bleaching of natural fibers in general. However, my bleaching process should be distinguished from processes such as are used to remove stains from fabrics. According to my bleaching process the actual whiteness and brightness of the fiber in its natural state is increased prior to its manufacture into paper or fabric. By brightness is meant the index of light reflected from a light source having a wave length of about 457 angstroms measured by the General Electric Brightness tester.

The bleaching process according to my invention'can stages} be carried out using conventional mechanical procedures and equipment such as is for example used for bleaching with chlorine or chlorine compounds, but using at least one gaseous fluorine or fluorine compound bleaching step.

In summary, the process controlling bleaching pollution according to my invention comprises treating the waste liquors and wash waters from my bleaching process with a reactive calcium compound, conveniently lime. The fluorine constituent will be partially or completely, depending upon interfering substances, precipitated as an insoluble calcium compound. This precipitate can then be removed by conventional liquid-solid separation processes; such as for example settling, centrifuging, filtering or the or by a combination of such separating processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Considering now my invention in greater detail, my invention can be conveniently described with reference to a conventional bleaching sequence.

A conventional bleaching sequence, for example, uses a series of stages which for covenience are designated by certain letters. For example, C is used to designate chlorination, E for extracting using caustic soda, H for sodium or calcium hypochlorite, D for chlorine dioxide and P for hydrogen peroxide. Thus, for example, a bleaching sequence for a sulphite pulp might be designated as CEH; C for chlorination, E for caustic soda extraction, and H for hypochlorite. A bleaching cycle for sulphate pulp might be designated as CEHED; i.e. chlorination, extraction, hypochlorite, extraction and chlorine dioxide. Another cycle might be six stage as CEHEHD; i.e. chlorination, extraction, hypochlorite, extraction, hypochlorite and chlorine dioxide. Another cycle might be CEDED. It is apparent that many other combinations may be used according to the properties desired in the bleached pulp and the starting material used in the process.

The C stage is used mostly for purification of the pulp by chlorinating the non-cellulosic materials present such as lignin. The E stage is used to solubilize and wash out chlorinated substances. The H stage is used to bleach and whiten the pulp and the D stage is added to further whiten the pulp Without substantially decreasing its strength. The P stage is sometimes used at, the end to reduce loss of brightness of the pulp upon aging.

' In each stage it is important to control the concentration or consistency of the pulp in suspension in water (called consistency by the industry), the percentage of the chemical used based upon the amount of fiber present, the temperature, the contact time, sometimes known as dwell time, and the pH.

In the first stage of my process, I can use a fluorine gas previously dissolved in water or passed directly into alone or in combination with gaseous fluorine or sodium fluoride. Further, any fluorine compound which will act, or can be suitably activated,;to act, as a fluorinating or oxidizing agent can'be used in the practice of my invention. The effects of the fluorine or its compounds upon the pulp are completed before the pulp is exposed to human contact. The fluorine bleaching treatment of my invention can be carried out by contacting the pulp with fluorine or a suitable fluorine" compound in an amount equal to 0.5 to 9 percent by weight based on the weight of the pulp at a temperature of about 90 to 160 F. for a period of 1 to 4 hours. Suitable bleaching agents which can be used in any process include fluorine, hydrofluorine acid and oxygen difluoride.

I prefer to designate my fluorine stage as F so that it can be written for example in a bleaching sequence as EEH meaning: fluorine, extraction and hypochlorite. Other examples where. a fluorine stage can be used in 4 place of a chlorine stage, according to my invention, include systems designated EEHED, EEHEHD, EEDED, EED and the like.

Particulraly useful forms of fluorine compounds are the oxy compounds, such as for example, oxygen difluoride (0P Oxygen difluoride can conveniently be prepared in a continuous process by bubbling fluorine into a 2% by weight solution of sodium hydroxide. The effluent from this process is an oxygen difluoride gas that can be passed directly into the pulp at any stage of my bleaching process. This is particularly useful in a final superbleaching stage such as is usually carried out by the use of chlorine dioxide. Even where chlorine dioxide is used stage other than the final stage, oxygen difluoride can be'used to replace it in that stage. Oxygen difluoride is a particularly suitable source of fluorine for the purpose of my invention since it decomposes into the components oxygen and fluorine at the temperatures of operation used in the normal bleaching system. Hereafter, I shall for convenience, designate the use of oxygen difluoride as the 0 stage.

The amounts of fluorine compounds to be added in my novel process will closely parallel the amounts of chlorine compounds which would be used in a chlorine system. Thus, if fluorine is added, it is used in the amount of 4 to 9%, usually about 7% by weight based on the pulp in the first stage, and the gaseous fluorine, added as such or generated in the pulp. Typically, a 3% pulp consistency is used. The bleaching reaction can, for example, be

carried out at the temperature of about to F.

with a retention time of about 1 to 4 hours, usually about 1% hours. The extraction stage with caustic soda can be carried out with a solution of /2 to 3% by weight caustic soda, preferably about 2 /2% caustic soda at a pulp consistency of preferably about 12% and a temperature of about F. to 180 F. for a period of preferably about 1 hour and an alkaline pH. In the dioxide stage of the process, the reaction would be carried out preferably with about 0.5% of the fluorine compound at preferably about 12% by weight consistency, at a temperature about 160 F. for a period of about 1 hour. In normal operation, the pulp would be washed between stages. If the pulp were to be treated with a peroxide, it would be a final treatment of the bleached pulp using about 1% usually without subsequent washing. More broadly this embodiment of my invention comprises the steps of: fluorination of the pulp with fluorine or a suitable fluorine compound; washing andextracting the pulp with a sodium hydroxide solution; treating the pulp with oxygen difluoride; and then washing and recovering the bleached product. -A more specific form of this general embodiment the fluorination is carried out using a gaseous hydrogen fluoride at a fluorine concentration of 2 to 9% and the oxygen difluoride treatment is carried out with .5 to 15% of the compound (i.e. oxygen difluoride) and a pulp consistency of 3 to 12%.

Considerable variation can be practiced in the amount of chemicals, consistency of the pulp, temperature, dwell time and the number and sequence of steps; and can be arranged to obtain the desired results as to pulp quality, brightness, opacity, strength, viscosity, permanganate number and the like, within, of course, the limits of the original pulp and the effectiveness of the chemical employed. Optimum operating conditions can be obtained by routine experimentation.

Groundwood pulps are bleached according, to the prior art process, in a single stage using about 1% sodium or zinc hydrosulphite at 2 to 5% consistency for about 1 to 2 hours, a temperature of about 110 to F. and a pH of about 4.5 to 6 for the zinc and a pH of about 6-7 for the sodium salt. The single stage can be supplemented by a subsequent stage of 1% sodium or hydrogen peroxide (stabilized with sodium silicate and magnesium sulphate) at 10 to 15% consistency and an alkaline pH for about 3 to'4 hours at 100 F. In accordance with my l 5 invention such pulps can be bleached by employing an F or an stage in substitution of or in conjunction with the prior art i'sta'ges. .3v

My process can also be used effectively to simultaneously bleach and remove (dissolve) siliceous compounds from the ncijiiies of grasses such as bagas' i'e, wheat straw and espartograsses and the like. These ,ffgrasses contain internal siliceous compounds within their nodes and thus can be rendered suitable for paper pulp only with extreme difficulty. However, according to my process, the gras ffiber 62m be bleached and the siliceous compounds rerrioved .byfthe addition of a fluorine step and/or the substitution of a fluorine step for one chlorine or alkaline steps used in the processes of the prior j art. Thus, the same sequeri of steps and conditions can be applied to effect the simultaneous bleaching and removal of nodular siliceous compounds as described above with respect to normal fiber bleaching, according to my prottiess.

Considering now the pollution control embodiment of myf invention. As noted previously, the waste liquors and wash water's, particularly from the bleaching of wood pulps, present a disposal problem since they usually contain unexhazt isted bleaching chemicals and also the materials removed from the pulp by the bleaching process. When such y'vaters contain a fluorine compound, as from one of the bleaching processes according to .my invention, their pollution potential can be substantially reduced or eliminated. Thus, according to my invention, these waste liquors and?" ash waters are treated with acalcium compound in. reactive form, conveniently lime. The fluorine constituent will be partially, or completely, depending upon the piesence of interfering substances, precipitated as a substantially insoluble calcium compound, which can-drag ddwn pollutants which may be in suspension within the waste liquor and wash water. The precipitation conditions such as for temperature and pI-I can be adjusted to further enhance this effect. The precipitate and any entrappejtd pollutants can be removed by conventional liquid-solid{separation means such as for example, settling, ceiitrifuging, filtering and the like; or by a combinatioriof such means.

In a fur'iher embodiment, an aluminum compound,

rmakers alum and the like, can be added to a separation of the precipitate. ium fluorides are considered non-toxic the oluble calcium precipitate can be used as a land fill aterial. Further where the calcium fluoride precipitate v substantially free, or can be conveniently freed, from bleach residues and other contaminants, the precipitate can be used as a source of fluorine for my bleaching processes.

Also, where the so treated waters contain useful material in solution, these materials can be recovered after thegtcalcium precipitate is removed by evaporation.

A further understanding of my invention can be had from the following illustrative, but non-limiting examples of specific, but non-limiting, modes of the invention.

EXAMPLE I 100 grams of a southern pine sulphate pulp having a viscosity of 58.0 centipoises at 3% consistency is treated with 5% by weight fluorine gas, added as such, at a temperature of 75 F. for a retention time of one hour. The pulp is then washed. The second stage is a treatment with 2.0% caustic soda for one hour at 12% consistency and 160 F. The pulp is then washed. The third stage is a treatment with 1.5% sodium hypochlorite for 2 /2 hours at 12% consistency, pH 10.0 and 95 F. The pulp is then washed. The fourth stage is extraction using 1.0% by weight caustic soda for 1 hour at 12% consistency and 160 F. The pulp is then washed. The fifth stage is a treatment with 1% by weight chlorine dioxide based on the pulp for several hours at 12% consistency and 160 F. The pulp is then given its final washing and the brightness willbe about 90.

In this example, oxydifluoride can be Cl'llOlillC dioxide in the fifth stage.

EXAMPLE II grams of a soft wood sulphite pulp is bleached by the FEH sequence with washings between the stages and at the entl; In the F stage 3% by weight fluorine is used at 3% consistency for /2 to 1 hourgat 60 F. with a of 2 -3, {and the E stage 0.5% caustic soda is used at 8% consistency for one hour at IF., and in the H stage 3% By weight available chlorine is used at 12% consistency for 2 /2 hours at 80 F. The 'pulp brightness will be about 87.

In making dissolving pulp, where a caustic soak stage is used, affinal treatment with peroxide is precluded by the presence of silicates in the peroxide solution. In place of the P stage an 0 stage can be employed.

EXAMPLE III 100 grams of an aspen semi-chemical pulp cooked by the cold soda process with a brightness of 39 is treated with 2% line hydrosulphite followed by l% by weight oxydifluoride based on the total amount to obtain a brightness of about 63.

EXAMPLE IV 100 grams a groundwood pulp with a brightness of 58 is treated with 1% oxydifluoride to obtain a brightness of about 65.

EXAMPLE V In this example, the liquors and wash waters from Example I are tir'eated with lime to precipitate the fluorine constituents. Sufiicient lime is added toiineutralize and precipitate the ,lfluorine constituents. The precipitate is removed by filtration.

What is clairtied is: I 1. A processjfor bleaching wood pulp which comprises the steps of: .1" {t (a) contacting the pulp with a quantity of oxygen difluoride j equal to about 0.5 to by weight, based on the weight of the pulp, for a suflicient time to bleach said pulp and to yield waste waters containing fluorine constituents, and 1 (b) treating said waste waters with a reactive calcium compound gwhereby insoluble precipitates of calcium and fluorine are formed.

2. The process of claim 1 wherein said pulpis contacted with said'i oxygen difluoride for about 1 to 4 hours at a temperatu of about 90 to F. and wherein said reactant celebr ty-compound is lime.

3. A processi itor bleaching wood pulp which comprises the steps of: N

(a) contacting the pulp with a quaritity, equals to about .5 to 9% by weight based on the weight of the pulp, of a bleaching agent selected from the group consisting of fluorine, hydrofluoric acid, and oxygen difluoride, for about 1 to 4 hours at a temperature of about from 60 to 160 F., whereby said pulp is bleached and whereby waste waters containing fluorine constituents are formed; l

(b) washing, and extracting the pulp with a A to 3% caustic soda solution at a temperatureof about 120 to F.,

(c) washing, and then treating the extracted pulp with oxygen-difluoride in an amount equal to about .5 to about 15% by weight, based on the weight of pulp, wherein the pulp is further bleached and wherein waste waters containing fluorine constituents are formed; and,

(d) treating the fluorine constituents containing waste waters from at least one of said fluorine type treating steps with a reactive calcium compound whereby used to replace 7 8 insoluble precipitates of calcium and fluorine are OTHER REFERENCES formed- Kirk-Othmerg Encyclopedia of Chemical Technology,

4. The process of claim 3 wherein said reactive cal- 2nd ed" Vol. 9 1m i e, New York (1 6) P 527-; cmm compound 18 Clark, J. T.: Determination of Lignin bji Hydrofiuo- 5. The Process of claim 3 where n at least a ortion 5 fine Acid, in Tappi VOL 45 No 4, April 1962, P of said insoluble precipitates of calcium and fluoride are 314, The chemfcal Age v01 57 a 11 recovered and recycled back to the bleaching process as Casey L R: i Paper I Intersciencg, e a source of fluorine containing bleaching agents. York (1 9 P- I References Cited UNITED STATES PATENTS 650,911 6/1900' Summers 16270 S. LEON BASHORE, Primary Examiner R. D. BAJEFSKY, Assistant Examiner U.S. Cl. X.R.

FOREIGN PATENTS 17,465 1902 Great Britain. 

